By Renee Maltezou, Michele Kambas and Lefteris Papadimas

ATHENS, Jan 5 (Reuters) - For a few tense hours on Sunday morning, Greek skies turned into a communications black hole. Air traffic controllers for Athens airport were guiding planes toward the runway when the usual radio chatter suddenly vanished - replaced by a piercing whistle.

It quickly became clear that controllers had lost contact with most aircraft in Greek airspace, including dozens of incoming flights, according to two controllers and an aviation official on duty at the time. 

Internet systems also appeared to fail across the board. Even the civil aviation authority's press office resorted to reading statements over the phone rather than sending by email.

The outage, which lasted several hours and affected most of Greece's airports, stranded thousands of travelers. Authorities have ruled out a cyberattack, but the cause remains unknown - and officials admit the systems didn't get fixed, they simply came back on their own.

"Suddenly communications went down. You could only hear a high-pitched whistle," said one controller, speaking on condition of anonymity. "The thing is, we don't know what caused it and how it ended. We want to find out the exact cause to ensure once and for all that this will not happen again."

Controllers managed to identify a couple of working radio frequencies in the tower, but not enough to maintain safe communication with pilots, a senior official said. Within half an hour, Greece took the unprecedented step of suspending flights into and across its airspace. 

Air traffic controllers from across the region stepped in to help. A controller in a neighbouring country said most communication with Greece was done over the telephone because the usual radios were down.

"We had a black hole in Greek airspace," said aviation safety expert Faithon Karaiosifidis. "Imagine if it had happened in the summer at the peak of the tourist season. The chaos."

OUTDATED INFRASTRUCTURE UNDER SCRUTINY

The incident has reignited calls to upgrade Greece's aviation infrastructure, which unions and experts say is outdated and underfunded after the country's 2009–2018 debt crisis.

The government insisted Monday that modernisation is underway and that current systems meet EU standards. The plan, which includes updating communication systems, is due for completion in 2028.

But last month, the European Commission referred Greece to the EU Court of Justice for failing to implement certain navigation procedures designed to boost safety in low-visibility conditions. It's unclear whether those measures would have made any difference on Sunday - but many worry reform is coming too late.

"This incident once again exposes the critical weaknesses of outdated and underfunded air traffic management infrastructure. Safety was maintained thanks to human expertise — but this cannot continue to compensate for systemic deficiencies," said Panagiotis Psarros, Chair of the Association of Greek Air Traffic Controllers. 

Experts say the problems go beyond ageing equipment. Radios dating back to the 1990s remain in use, and staffing shortages persist despite a tourism boom that brings millions of visitors to Greece each year.

"The old technical equipment and the lack of personnel in air traffic controllers and electronic technicians ... create a bottleneck," said Karaiosifidis.

In September, unions protested by limiting flight arrivals they said exceeded permitted limits, causing delays. They have now threatened to do the same again.

(Writing and additional reporting by Edward McAllister; Editing by Ros Russell)